This project has two major objectives: to determine how current learning is affected by previous learning, and to determine how certain internal stimulus states of the organism produced by reward events, these states being conceptualized as memories, regulate learning. Both objectives may be pursued using highly similar experimental designs. The favored experimental design is this: following some original learning condition, a shift to some new learning condition is provided. Major, but not exclusive emphasis, is invested in the second learning condition, sometimes called the shift condition. The shift condition may consist of either better reward (larger reward, a higher percentage of reward or a short delay of reward, or some combination of these), or worse reward, the limiting case reward, of course, being nonreward. Several possible phenomena in the shift condition are of major concern: positive contrast effects, negative contrast effects and, of course, resistance to extinction. Emphasized in our laboratory relative to other laboraories are the performance effects of regular or recursive reward schedules. Often, findings obtained using regular schedules suggest that non-regular schedule findings are not sufficiently general to support correct inferences.